Master your DSLR: in-camera metering

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INCIDENT LIGHT The light falling on the subject is reflected towards the camera

 

CAMERA METER The camera measures the intensity of the light entering the lens

 

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LIGHT METER An incident light meter can be used to measure the light falling on the subject

 

MANUAL EXPOSURE The exposure is then manually set on the camera

 

YOUR CAMERA’S METERING system measures the intensity of the light reflected by the scene that you’re photographing and determines the appropriate settings that will be required to record that light.

This is known as a ‘through the lens’ reflective light reading. You can also take an incident light reading using an external light meter, and set the suggested exposure manually on the camera. Incident light meter readings have the potential to be more accurate, but the process isn’t as convenient as using your camera’s meter.

Lightly tapping the camera’s shutter release button activates the meter, and a suggested aperture and shutter speed combination — one that should give a ‘correct’ exposure, based on the lighting and the ISO sensitivity — is displayed in the viewfinder. There is a range of factors that can lead to the exposure not being quite as correct as it should be, which we’ll get to shortly.

The metering sensors found in today’s digital cameras typically do more than just register the intensity or luminance of the light coming through the lens. They can also take into account the colours in the scene or subject being photographed, and can prioritise the exposure for the parts of a picture that are in focus. These days, metering sensors are densely packed with pixels and are effectively more like low-resolution imaging sensors; in some high-end cameras, they’re being used in conjunction with the camera’s autofocus system to improve the detection and tracking of objects using the colour information, as well enabling face detection through the viewfinder.

Your digital camera might have a single metering sensor, but there’s a range of different metering modes or patterns that you can choose from. These determine how much of the scene is metered and whether the camera makes any adjustments to the meter reading. When the camera is set to an advanced shooting mode, such as Aperture Priority, you can choose an appropriate metering mode. The default mode is ’pattern’ metering, commonly referred to as Evaluative, Matrix or Multi-pattern. In this mode, the camera splits the image into a series of small zones from which it takes individual meter readings. These readings are then compared before the camera suggests an exposure setting that should give a balanced result. It effectively applies its own exposure compensation to take account of variations in brightness and where it determines the subject is.

THE METERING SENSORS IN TODAY’S CAMERAS TYPICALLY DO MORE THAN JUST REGISTER THE INTENSITY OF THE LIGHT THROUGH THE LENS

The other metering modes that you’ll find on your camera don’t analyse the scene in quite the same way, and are best reserved for occasions when you have the time and experience to take advantage of them. Take spot metering, for example. This only takes around 1.5–3% of the picture into account, making it unbeatable for taking precise readings from small objects in a scene, or when you want to stop large areas of dark or light tone from fooling the meter. The drawback is that the spot meter is configured to produce a mid-tone exposure value. Point it at a mid-tone subject, such as lush green leaf, and you should get an accurate exposure. But point it at an object that’s either brighter or darker than mid-tone and you’ll end up with an image that’s either underexposed or overexposed. (See ‘Metering problems’ at the top of the following page.)

Pattern metering doesn’t always get it right either. It can come unstuck in high-contrast conditions for instance, and it won’t take into account any creative effect that you’re trying to achieve. In both situations, you can use exposure compensation to fine-tune the result. Shooting in the RAW format will enable you to tweak the exposure when you process the result, but you might as well try and get it close to perfect in-camera.

METERING MODES AT A GLANCE

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HOW EACH OF THE MAJOR METERING PATTERNS ON A DIGITAL CAMERA WORKS...

PATTERN

The default metering mode measures light distribution across the whole frame, and often takes colour and focus data into account, too. It then calculates whether the scene needs to be made brighter or darker before it arrives at a suitable exposure for the shot.

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With pattern metering, the shape of the graph changes with each scene.

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Your current focus point is one of the criteria used by pattern metering to work out the best exposure.

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CENTRE-WEIGHTED

This mode measures the light across the whole picture area, but strongly biases the resulting reading to the centre of the viewfinder. On more advanced digital cameras, you can change the size of this central area, and hence affect the overall bias.

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This graph is higher in the middle, where it’s heavily ‘weighted’.

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The exposure is based mainly on the central part of the image, although the outer parts have an influence, too.

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SPOT/PARTIAL

Spot metering only measures the intensity of light over a small circular area around either the active AF point or the centre one, depending on the camera. The Partial mode available on some cameras is similar, but measures a slightly larger area.

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This graph shows how spot metering concentrates around the AF point.

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The small area around the AF point gives an indication of the region covered by spot metering.

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AVERAGE

This is a variation on the centre-weighted mode that’s only available on some cameras. It measures light evenly across the whole frame, which makes it the least sophisticated metering system of all, but it can be the easiest for experts to interpret.

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The graph for average metering is flat, as all areas are treated equally.

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The light across the whole frame is measured as a single value. It’s a crude method, but it can be useful.

REAL-WORLD METERING

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THE PROS AND CONS OF EACH MODE, AND THE SUBJECTS THEY SUIT.

PATTERN

1 This is just a representation of the zones in pattern mode. In reality, the scene may be split into hundreds or even thousands of zones.

2 The central area will not necessarily get priority over other areas — it depends on the light distribution across the rest of the scene.

3 It can tend to give priority to dark areas to prevent underexposure, although it’s still a good choice for quickfire, everyday photography.

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CENTRE-WEIGHTED

1 This can be a good choice for portraits, particularly backlit ones, and photos where the subject is towards the middle of the picture.

2 The outer areas play a smaller part in the exposure measurement, but they are still taken into account.

3 Centre-weighted doesn’t apply its own compensation before giving you an exposure, so you may need to apply more substantial exposure adjustments to get the desired result.

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SPOT/PARTIAL

1 Switching to the spot metering option means that you can avoid large areas of bright or dark tones affecting the meter reading.

2 On beginner-friendly cameras, the spot zone is slightly larger to allow more leeway for errors.

3 The spot is not always in the centre of the frame and may be linked to your manually selected AF point, making it possible to meter off-centre subjects. Keep this in mind when metering.

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AVERAGE

1 The light across the whole frame is averaged into a reading, making it a good option when the scenes contain a balanced blend of tones.

2 A word of caution: average meter readings are very susceptible to small bright areas in the scene.

3 The advantage to averaging is that it doesn’t take long to become familiar with how it will react in different situations, making exposure adjustment straightforward.

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ESSENTIAL CONTROLS

DISCOVER THE KEY CAMERA SETTINGS THAT ENABLE YOU TO SET AND OVERRIDE THE METERING SYSTEM.

01 METERING MODE

Pattern metering is set by default in the automatic or scene modes. To change the metering mode, you need to be in the semi-automatic or manual exposure modes.

02 HISTOGRAM

Don’t trust the image on the rear screen — use the playback histogram to help judge the exposure more accurately. In Live View mode, you’ll be able to call up a real-time histogram, too.

03 COMPENSATION

You can use exposure compensation to correct exposure problems. (See the opposite page.) Some cameras let you dial this in directly; others require a button press first.

04 DYNAMIC RANGE

Your camera might not be able to hold detail throughout the scene if the difference between the highlights and shadows is extreme, such as in strongly backlit shots like this one.

EXPOSURE LOCK

Keep the shutter release half-pressed to lock the exposure, enabling you to take a meter reading from a different part of the scene, then recompose the shot. Your camera is likely to have a separate Exposure Lock button on the rear of the body as well.

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DARK SUBJECT

 

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Little light reflected

 

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Camera overexposes

 

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Histogram indicates a mid-tone

 

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MID-TONE SUBJECT

 

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Average amount of light reflected

 

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Normal exposure

 

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Histogram indicates a mid-tone

 

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BRIGHT SUBJECT

 

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Lots of light reflected

 

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Camera underexposes

 

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Histogram indicates a mid-tone

 

METERING PROBLEMS

IF THE METER IN YOUR CAMERA IS HIGHLY ADVANCED, WHY DOES IT GET THINGS WRONG?

The reason why your camera doesn’t always nail the correct exposure is that it meters the light reflected by the scene — and some subjects reflect more or less light than others. Camera meters are designed for 18% reflectance — in other words, if a subject reflects about 18% of the light that falls on it, the camera should produce a ‘correct’ exposure. Mid-tone subjects such as grass and grey pavements reflect roughly 18% of the light, and many scenes average out close to this value. But when it’s faced with an overall dark subject, the camera will read this as a mid-tone subject that’s not receiving enough light, and will extend the exposure — making the subject look too bright. With a bright subject, the camera will see a mid-tone subject that’s receiving too much light and reduce the exposure, making the subject look too dark.

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EXPOSURE COMPENSATION

When you’re shooting in one of the camera’s semi-automatic exposure modes, such as Aperture Priority, Shutter Priority or Program, you can fix metering problems using exposure compensation. This allows you to increase or decrease the exposure value of subsequent pictures you take; use plus exposure compensation to make them brighter and minus exposure compensation to make them darker. Remember to cancel this once you’ve finished taking pictures: exposure compensation doesn’t reset itself, even when you turn the camera off.